Nearly 25% of the total world wide energy consumption in 2005 was used in the US. A third of the 100.2 quadrillion British Thermal Units (‘quads’) total US energy consumption is used for electricity. About 25% of the electricity in the US is used for lighting. About 8% of the total worldwide energy used is transformed into artificial lighting. Currently, inefficient incandescent lighting (efficiency ˜5%) is being replaced by fluorescent lighting (compact fluorescent lights CFL at an efficiency ˜25%) to reduce our carbon footprint.
The quantum efficiency in light-emitting devices based on photon conversion is the ratio of the number of emitted photons divided by the number of absorbed photons. The potential increase of the efficiency of fluorescent lamps from 25 to 50% by using multi-photon processes would translate into commensurate energy savings and provide a technology platform before advanced light-emitting diode (LED) technologies can significantly penetrate the market.
A light-emitting phosphor consists of a host lattice into which activator ions such as rare earths are doped at a few mol %. Certain lattices are self-activating and emit light without the presence of activators. Particular electronic energy levels of the activator ions can be populated either directly by excitation or indirectly via energy transfers inducing luminescence and non-radiative decay processes. Luminescence is a down-conversion process in which a higher energy photon (typically from the UV region between 200-400 nm) is absorbed and a lower energy photon is emitted in the visible region of the electromagnetic spectrum between 400 and 750 nm. This process is also referred to as a Stoke's process, and the difference in wavelength as the Stoke's shift. Alternatively, “anti-Stokes” processes or infra-red-to-visible light ‘up-conversion’ phosphors occur where two or more infra-red photons are absorbed to produce one visible photon at a higher energy.
Generally, two types of activator ions exist: those that interact weakly with the host lattice via their f-electron energy levels and those that interact strongly with it via their s2 and/or d-electrons. In the first case, rare earth ions (REn+) allow optical transitions between their different discrete energy levels resulting in narrow line emissions as seen, for example, in Y2O3:Eu3+, whereas in s2 ions such as Pb2+ or Sb3+ and transitions metals such as Mn2+ broad bands of radiation are emitted. The host materials must be optically transparent since excitations should only take place at the activator sites. Therefore, ions with rare gas electron configurations or closed electron shells are preferred.
About 90% of all artificially-generated photons come from discharge lamps generating UV light, the most widespread being those based on Hg plasma with 75% conversion efficiency. The low pressure Hg plasma has three main emission lines at 185, 254 and 365 nm. If the pressure is increased above 1000 torr, a continuum between 250 and 350 nm is created by the Hg discharge. Low-pressure Hg discharge lamps coupled with a phosphor coating have an energy conversion efficiency of about 25-30%.
A phosphor emits light in a narrow frequency range, unlike an incandescent filament, which emits the full spectrum, though not all colors equally, of visible light. Mono-phosphor lamps emit poor quality light. Their colors look bad and are inaccurate. The solution is to mix different phosphors, each emitting a different range of light. Properly mixed, a good approximation of daylight or incandescent light can be reached. However, every extra phosphor added to the coating mix causes a loss of efficiency and increased cost. Good-quality consumer CFLs use three or four phosphors—typically emitting light in the red, green and blue spectra—to achieve a “white” light with color-rendering indices (CRI) of around 80 although CFLs with a CRI as great as 96 have been developed. (A CRI of 100 represents the most accurate reproduction of all colors; reference sources having a CRI of 100, such as the sun and incandescent tungsten lamps, emit black body radiation.)
An example of a visible light phosphor is the co-doped fluoro-chloro-apatite Ca5(PO4)3(F,Cl):Sb3+,Mn2+, where the blue emissions from the Sb3+ and the orange emissions from the Mn2+ activators result in white light at about 80 lm/W. Another way to produce white light is by combining the red, green and blue emissions (“tricolor concept”) using, for instance, BaMgAl10O17:Eu (λmaxem=450 nm), (Ce,Gd,Tb)MgB6O10 (λmaxem=542 nm) and Y2O3:Eu (λmaxem=611 nm) phosphors resulting in white light emission at about 100 lm/W.